1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device, which is capable of preventing a touch defect and gravity defect and has a stable structure resistant to a push test, and a method for manufacturing the same.
2. Discussion of the Related Art
The development of information-dependent society creates a strong demand for various types of display devices. To fulfill the demand, efforts have recently been made to research flat panel display devices, such as liquid crystal display (LCD) devices, plasma display panel (PDP) devices, electro-luminescent display (ELD) devices, and vacuum fluorescent display (VFD) devices. Some types of such flat panel display devices are being practically applied to various appliances for display purposes.
Among the above mentioned display devices, in particular, LCD devices have been used as a substitute for cathode ray tube (CRT) devices because of its outstanding characteristics and advantages, for example, superior picture quality, lightness, thinness, and low energy consumption. Thus, LCD devices are currently most widely used. Various applications of LCD devices are being developed in association with not only mobile image display devices such as monitors of notebook computers, but also monitors of television sets to receive and display broadcast signals, and monitors of laptop computers.
Successful application of such LCD devices to diverse image display devices depends on whether or not the LCD devices can realize desired high picture quality including high resolution, high brightness, large display area, and the like, while maintaining desired characteristics of lightness, thinness, and low power consumption.
A general LCD device includes first and second substrates that are bonded to each other with a certain space therebetween, and a liquid crystal layer formed between the first and second substrates.
More specifically, the first substrate includes a plurality of gate lines that are arranged in one direction while being uniformly spaced apart from one another, and a plurality of data lines that are arranged in a direction perpendicular to the gate lines while being uniformly spaced apart from one another. The gate lines and data lines define pixel regions. The first substrate further includes pixel electrodes arranged at the respective pixel regions, and thin film transistors respectively formed at intersections of both the gate lines and data lines. The thin film transistors serve to apply data signals of the data lines to each pixel electrode in response to signals applied to the gate lines.
The second substrate includes a black matrix layer for blocking incidence of light to regions other than the pixel regions, R, G, and B color filter layers respectively formed at regions corresponding to the pixel regions and adapted to express color tones, and a common electrode formed on the color filter layer and adapted to reproduce an image.
In the LCD device having the above described configuration, liquid crystals of the liquid crystal layer that is formed between the first and second substrates are oriented by an electric field between the pixel electrodes and the common electrode. The quantity of light passing through the liquid crystal layer is regulated based on the orientation degree of the liquid crystal layer, to display an image.
The LCD device described above is called a “twisted nematic (TN) mode LCD device”. The TN mode LCD device has a drawback of a narrow viewing angle, and therefore, an in-plane switching (IPS) mode LCD device has been developed to overcome the drawback of the TN mode LCD device.
In the IPS mode LCD device, a pixel electrode and a common electrode are formed on a first substrate at each pixel region of the first substrate such that the pixel electrode and common electrode extend parallel to each other while being spaced apart from each other to generate an in-plane electric field (horizontal electric field), thereby allowing liquid crystals of a liquid crystal layer to be aligned with the in-plane electric field.
Meanwhile, spacers are provided between first and second substrates of the LCD device having the above described configuration, to maintain a predetermined gap for the liquid crystal layer.
The spacers are classified into ball spacers and column spacers based on their shapes.
The ball spacers have a spherical shape, and are scattered on the first and second substrates. Even after the first and second substrates are completely bonded to each other, the ball spacers are relatively free in movement, and have a small contact area with respect to the first and second substrates.
The column spacers are formed during an array process of the first or second substrate. The column spacers are fixedly mounted on a selected one of the substrates, and have a circular cylinder shape with a certain height. Accordingly, the column spacers have a relatively large contact area with respect to the first and second substrates as compared to the ball spacers.
Hereinafter, a conventional LCD device having column spacers will be explained with reference to the accompanying drawing.
FIG. 1 is a sectional view showing the conventional LCD device having column spacers.
As shown in FIG. 1, the LCD device having column spacers includes: first and second substrates 30 and 40 arranged to face each other; at least one column spacer 20 formed between the first and second substrates 30 and 40; and a liquid crystal layer (not shown) filled between the first and second substrates 30 and 40.
The first substrate 30 includes: a plurality of gate lines 31 and data lines (not shown), which are arranged perpendicular to each other and adapted to define pixel regions; thin film transistors (TFT) formed at intersections of the gate lines and data lines; and pixel electrodes (not shown) arranged at the respective pixel regions.
The second substrate 40 includes: a black matrix layer 41 formed at regions other than the pixel regions; color filter layers 42 having a stripe pattern and formed corresponding to the pixel regions that belong to vertical lines parallel to the data lines; and a common electrode or overcoat layer 43 formed over the entire surface of the second substrate 40.
The column spacer 20 is formed corresponding to a certain position on a top of the relevant gate line 31.
Additionally, the first substrate 30 further includes: a gate insulation layer 36 formed over the entire surface of the first substrate 30 including the gate lines 31; and a protective layer 37 formed over the gate insulation layer 36.
FIGS. 2A and 2B are a plan view and a sectional view, showing a touch defect of the LCD device having the column spacer.
As shown in FIGS. 2A and 2B, in the case of the LCD device having the column spacer as stated above, when a surface of a liquid crystal panel 10 is continuously touched with a finger or object along a certain direction, a spot is generated at the touched portion. The spot may be called a “touch spot” because it is generated by a touch action, or may be called a “touch defect” because it is observed on a screen.
The reason to cause the touch defect in the LCD device having the column spacer is that the column spacer 20 has a large contact area with a first substrate 1 as compared to ball spacers. Therefore, it suffers from a larger frictional force to cause the touch defect. More specifically, as shown in FIG. 2B, since the column spacer 20 has a larger contact area with the first substrate 1 than ball spacers, a large frictional force caused by the larger contact area prevents the rapid restoration to the original state when the first and second substrates 1 and 2 are shifted relative to each other by a touch action. This results in the long-lasting spots.
FIG. 3 is a sectional view showing a gravity defect of the LCD device.
As shown in FIG. 3, if the LCD device, in which the liquid crystals 3 are filled between the first and second substrates 1 and 2 and the column spacer 20 is formed between the first and second substrates at a predetermined position, is placed in a vertical direction for an extended period of time while being maintained at a high temperature, the liquid crystals expand due to the high temperature, thereby causing a cell gap extending beyond a height of the column spacer 20. As a result, gravity causes the liquid crystals to move to a lower portion of the LCD device, creating a swollen portion at the lower portion. This swollen portion is called a “gravity defect”.
The conventional LCD device having the column spacer as stated above has the following problems.
First, a contact area between the column spacer and the substrate is excessively large. Accordingly, when the substrate is shifted by a touch action, the large contact area causes a large frictional force, thereby preventing the rapid restoration to the original state, and resulting in long-lasting touch defects.
Second, if a liquid crystal panel having the column spacer is placed in a vertical direction for an extended period of time at a high temperature, the liquid crystals expand due to the high temperature, causing a cell gap extending beyond the height of the column spacer. As a result, gravity causes the liquid crystals to move to a lower portion of the liquid crystal panel, creating a swollen portion at the lower portion. The swollen portion is observed as an opaque portion.
Third, when the LCD device is subjected to a push test for the inspection of durability prior to shipping, a predetermined pressure is applied to a certain region of the LCD device. In this case, if the column spacer is insufficient to maintain a cell gap between upper and lower substrates, the column spacer may be crushed, resulting in what is known as a “imprinting spot” at the position of the column spacer.